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Theses

Understanding of chemical reactions involved in pigment discoloration, in particular in mercury sulfide (HgS) blackening

Abstract : Pigments colour can sometimes evolve, unexpectedly, from the painter's or curator's point of view. The most common degradation of red mercury sulfide (α-HgS), used in a cinnabar natural form or in a vermilion synthetic form, is turning into black or silver-gray. This phenomenon is rather capricious and does not happen systematically. Since Antiquity, cinnabar was often used in paintings, even if it was known to suffer from degradation under the influence of sunlight and humidity. Analyses of original works of art presenting mercury sulfide in their pictorial layers allowed identifying different degradation compounds, such as calomel (Hg2Cl2) and different polymorphous phases of Hg3S2Cl2. Furthermore, artificial ageing experiments were used to reproduce the formation of some of these products on model samples and confirm light and chlorine as two necessary parameters to induce the presence of these alteration products. Pourbaix diagrams and simulations obtained from thermodynamics calculation appeared to be consistent with the evolution of the composition of α-HgS model samples during artificial ageing experiments. The chemical characterization of the alteration products was rather challenging mainly because of the micrometric size of the alteration (multi-)layers. Beyond the different analytical techniques available, synchrotron-based micro X-ray diffraction, micro X-ray fluorescence and micro X-ray absorption spectroscopy proved to be very helpful for the identification and localization of both original and altered compounds. These researches confirmed preliminary hypothesis on factors influencing the alteration of mercury sulfide and explain the conditions in which these degradations occur in order to avoid them in the future.
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https://tel.archives-ouvertes.fr/tel-00805147
Contributor : Marie Radepont <>
Submitted on : Thursday, March 28, 2013 - 11:48:06 AM
Last modification on : Tuesday, December 8, 2020 - 11:40:03 AM
Long-term archiving on: : Saturday, June 29, 2013 - 4:01:51 AM

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  • HAL Id : tel-00805147, version 1

Citation

Marie Radepont. Understanding of chemical reactions involved in pigment discoloration, in particular in mercury sulfide (HgS) blackening. Analytical chemistry. Université Pierre et Marie Curie - Paris VI, 2013. English. ⟨tel-00805147v1⟩

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